I would like to know how I can change on site library Dispatch the size of single post feature image, right now is showing full image on top and I want to show image only on content area not sidebar area, how I can do this?

go to Appearance > Elements. You’ll see a couple of Header Elements, you need to Edit and save as draft ( or delete ) the Single Post header element. The site will then display the default featured image and post meta etc.

You may want to have a quick read of the post included in the original Dispatch site, it explains how the site as built and how to make changes to it: